Three hypotheses concerning plasticity of extravisual afferents of the fish tectum are proposed. Two closely related teleosts, the blind cave fish (Astyanax hubbsi), and its sighted ancestor (Astyanax mexicanus), will provide ideal experimental subjects, since major differences are known to exist in the sensory input to their tecta yet they are completely interfertile. The cave fish has a well developed somatosensory representation in its tectum, while its ancestor has an optic tectum similar to other teleosts (such as the goldfish, which will be used for control purposes). Three related sets of experiments are proposed in order to test these hypotheses. In the first the sighted form (A. mexicanus) will be enucleated in an attempt to induce a cave fish-like tectum. In the second, we will attempt to demonstrate plasticity in the topography of tectal afferents in the two forms, similar to that which has previously been shown in goldfish (field compression). These two studies will utilize electrophysiological methods. The third will use anatomic methods to compare the pattern of tectal afferents in A. mexicanus and hubbsi. These proposed studies will: 1.) provide information about an unusual evolutionary adaptation, 2.) provide comparative data about the vertebrate visual system, and 3.) extend the findings of previous studies on the developmental mechanisms underlying plasticity of neural connections.